Ironman 70.3 Relay Rules

How flexible are the rules to switch out members of an IM 70.3 relay team? Does the team captain have to do a leg of the race? How late can team members be added or switched out?

Here’s my situation: My wife has to skip an upcoming 70.3 due to a running related injury. Deferral is not an option. She would like to do the same race next year, but she’s worried about this running injury returning.

Even though it’s more expensive, I had an idea to register as a relay team with just the two of us and her as the team captain. If she feels fine, she’ll do the entire event. If she can’t run, she’ll swim & bike then I’ll do the run. Or worst case, I would do the entire event (which is unlikely). We would figure this out before the day of the event.

I assume we would both get athlete bracelets at registration and only 1 timing strap, so I don’t see how IM would know or care who actually did the legs of the relay.

Is the above doable?